SV-POW! … All sauropod vertebrae, except when we're talking about Open Access

Thinking about sauropod skin

March 22, 2018

Someone on Facebook asked whether sauropods had subcutaneous fat, and by the time my answer hit five paragraphs I thought, “The merciful thing to do here is blog this and link to it.” So here are some things to keep in mind regarding the integumentary systems of sauropods.

Emu dissection at UC Santa Cruz back in 2004. Note the fat pad on the chest and how it abruptly comes to an end.

Sauropods may have had some subcutaneous fat – we can’t rule it out – but it probably wasn’t broadly distributed as it is in mammals. In the interaction of their air sac systems with connective tissue, sauropods were probably a lot like birds. Most birds don’t have subcutaneous fat all over their bodies. Instead, they have subcutaneous air sacs (or pneumatic diverticula) over parts or all of their bodies – in pelicans these are like bubble wrap under the skin, presumably for impact padding and insulation (Richardson 1939, 1943). The diverticula go everywhere and most places they go, they replace adipose tissue, even the harmless bits of fat between muscles that are basically the body’s packing peanuts (broiler chickens don’t count here, they’ve been artificially selected to be radically unhealthy). We suspect that sauropods had subcutaneous diverticula because so many other aspects of their pneumatic systems correspond to those of living birds (see the discussion in Wedel and Taylor 2013b for more on that).

Contrast the narrow line of adipose tissue down the ventral midline with the almost-completely-lean hindlimb.

That’s not to say that birds don’t have subcutaneous fat, just that it tends to be highly localized. Back in grad school I got to help dissect an emu (link) and a rhea (one, two), and in both cases the fat was concentrated in two places: huge paired fat pads over the pelvis, like big lozenges, and a concentration over the sternum with extensions along the ventral midline from the base of the neck to the cloaca. It was weird, the fat would be present and then it would just stop, like somebody flipped a switch. We pulled 18 lbs of fat off a 102-lb emu, so it wasn’t a trivial part of the body composition. IME, even relatively fatty birds like ducks tend to have the fat start and stop abruptly, and again, the fat deposits tend to be concentrated on the breast and tummy and over the hips.

A lot of lizards and crocs and even some turtles carry fat deposits in their tails, and that is one aspect of sauropod anatomy that is definitely un-bird-like. So some sauropods might have had fat tails.

We can be pretty sure that at least some sauropods had thick skin. Osteoderms (armor plates) from Madagascar show that the bits that were embedded in the skin could be up to 7cm thick, so the surrounding skin was at least that thick and possibly even thicker (Dodson et al. 1998). And that was most likely on Rapetosaurus, which was not a huge sauropod. So giant sauropods might have had even thicker skin. Maybe. Remember that big-ass-ness (here arbitrarily defined as 40+ metric tons) evolved independently in:

They probably didn’t all get there looking the same way, beyond sharing the basic sauropod bauplan.

I’m too lazy to write about the fossil evidence for scaly skin and keratinous spines in sauropods – see this post and the references therein.

One final thing to think about is scar tissue. The scar tissue on the chest of a male elephant seal can be up to 5cm thick. Some sauropods might have had calluses or patches of scar tissue in predictable places, from combat, or habitually pushing down trees with their chests or tails, or doing whatever weird things real animals do when we’re not looking.

So in the toolbox of things to play with in reconstructing the integument of sauropods, we have thick skin, keratinous spines, osteoderms, fat pads (possibly concentrated over the hips and shoulders or on the tail), subcutaneous diverticula, calluses, and scar tissue. And that’s just the stuff we have found or reasonably inferred so far, barely 150 years into our exploration of animals we know mostly from bits and bobs, whose size means they mostly got buried in big sediment-dumping events that would not preserve delicate integumentary structures. Give us a millennium of Yixian Formations and Mahajanga Basins and Howe Quarries and the picture will probably change, and the arrow of history dictates that it will change for the weirder.

Likely? Probably not. But roll the evolutionary dice for 160 million years and you’ll get stranger things than this. Recycled from this post.

Finally, and related to my observation about big-ass-ness: sauropods were a globally-distributed radiation of animals from horse-sized to whale-sized that existed from the Late Triassic to the end of the Cretaceous. The chances that all of them had the same integumentary specializations, for display or combat or insulation or camouflage or whatever, are pretty darned low. Support weird sauropods – and vanilla ones, too.

Almost immediate update: I’ve just been reminded about Mark Witton’s excellent post on dinosaur fat from a couple of years ago. Go read that, and the rest of his blog. I’m sure I missed other relevant posts at other excellent blogs – feel free to remind me in the comments.

This is great stuff. I wonder whether some sauropods might have shed their skin in big patches, like lizards, elephant seals, belugas, etc. I imagine the beasts rubbing against convenient rocks and trees, leaving tarp-sized sheets of exuvium littering the landscape…

On another note: do you guys know if any new research is forthcoming on the “Amphicoelias brontodiplodocus” specimens? IIRC, at least one of the specimens was acquired by a public institution (in Mexico, I think).

Haven’t heard anything about “Amphicoelias emtpyhypensis” in ages, but I anticipate that if it does make it into a public institution, it will eventually be synonymized with Kaatedocus – at least, that’s what it looks like to me.

[…] Mike ruled March with a flurry of posts, including a couple worth revisiting on how grant funding is awarded and on the state of play vis-a-vis Big Publishing. Also (and uncharacteristically) Mike posted on appendicular bones of birds, both skinny and fat. It was left to me to represent for sauropods, with posts on the cervical vertebrae of Alamosaurus and Suuwassea and some noodling about sauropod skin. […]